Method and system for closed loop power control in wireless systems

ABSTRACT

The present invention relates to a method and system for closed loop power control in a mobile communication system. Error is determined between a target signal-to-interference (SIR) and a received SIR. A power control scheme at a power controller of a base station determines a power change that is proportional to the error. Gain of the power controller is selected to achieve minimum error at a subsequent time instant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the area of power control in wireless systemsand more particularly the present invention relates to a method andsystem for providing closed loop power control in narrow band or spreadspectrum wireless systems.

2. Description of the Related Art

In the field of wireless communications, several technologies exist forcontrolling communications between a mobile station, such as a cellulartelephone or personal communication system (PCS) handset, and a wirelessbase station. In a narrow band wireless network, time and frequencyslots are partitioned in order to avoid material interference betweenusers. Users can share the same time or frequency slot provided they aresufficiently far apart and such users are known as co-channel users. Theissue of power control arises because the co-channel users interferewith each other. Carrier/Interference (C/I) balancing schemes have beendescribed in which C/I is balanced to provide a distribution of theinterference such that all users have the same C/I or have the samecarrier to interference ratio (CIR), i.e. the ratio of the power levelof a desired signal received at a given location to the power level ofall other received signals of the given location, orsignal-to-interference ratio SIR, see Zander, “Performance of optimumtransmitter power control in cellular radio systems,” IEEE Trans. Veh.Technol., Vol. 41, pp. 57-62, February 1992; and, “Distributed CochannelInterference Control in Cellular Radio Systems,” IEEE Trans. Veh.Technol., Vol. 41, pp. 305-311, August 1992; and Grandhi et al.,“Centralized Power Control In Cellular Radio Systems,” IEEE Trans. Veh.Technol., Vol. 42, No. 4, pp. 466-468, November 1993; and, “DistributedPower Control In Cellular Radio Systems,” IEEE Trans. Comm., Vol. 42,No. 2/3/4, pp. 226-228, February/March/April 1994.

In code division multiple access (CDMA) systems, users share the samefrequency all the time by using a specific spread spectrum pseudonoisecode for each user. Fundamental to the CDMA system is power control. TheCDMA system is an interference limited system in the sense that thesystem capacity, related to the number of simultaneous calls, is afunction of the maximum amount of interference that the system cantolerate. In order to maximize the system capacity, the transmittedpower of each mobile unit is controlled so that its signal arrives atthe cell site with the minimum allowable SIR. Power control is used tomitigate the “near/far problem” preventing users that are geographicallycloser to the base station from “over-powering” users that are fartheraway.

Open loop and closed loop power control schemes have been described. Thegoal of open loop power control is to adjust the transmitted poweraccording to changes in the received power. In the open loop powercontrol method according to IS-95, the mobile station uses the measuredtotal received power along with typical values of certain base stationparameters to get a rough estimate of the transmission loss between theunit and the base station. Based on these measurements, the forward linktransmission loss is estimated and used to determine the proper openloop power control setting for the mobile station transmitter. Themobile station's transmit power is adjusted to match the estimated pathloss, to arrive at the base station at a predetermined level. All mobilestations use the same process, and ideally their signal will arrive withequal power at the base station. See “Telecommunications IndustryAssociation/Electronic Industries Association (TIA/EIA) Interim StandardIS-95 series including IS-95A and IS-95B, entitled “Mobile Station—BaseStation Compatibility Standard for Dual-Mode Wideband Spread SpectrumCellular System.”

U.S. Pat. No. 6,101,179 describes a method for open loop power controlin a CDMA communication system including calculating in the base stationa base station pilot transmit power and a base station receivedsensitivity value. The base station transmits the pilot transmit powervalue and the receiver sensitivity value to the mobile base station. Themobile station calculates a mean output power in response to the basestation pilot transmit power value and the base station receiversensitivity value. The open loop control can cope only with very slowshadow fading.

In closed loop power control, the base station measures the relativereceived power level of each associated mobile station and compares itto an adjustable threshold. A determination is made to transmit a powerup command or a power down command to the mobile station. The mobilestation can make received adjustment commands with open loop estimatesto obtain the final value for transmitted power. The goal of the closedloop power control is to provide rapid corrections to the open loopestimate in order to obtain the optimum transmit power.

A Quality-of-Service (QoS) based closed-loop power control performsbetter than the power-level based approach. The quantity used to measureQoS is the SIR, or Eb/I₀. The IS-95 power control system is an up/downhard decision type. If the actual SIR is lower than an SIR target value,the transmission power is raised by a fixed step size, such as 1.0 dB,or 0.5 dB. Alternatively, if the actual SIR is higher than an SIR targetvalue, the transmission power is reduced by a fixed step size, set as1.0 dB, 0.5 dB. A conventional power control scheme is called theDistributed Constrained Power Control (DCPC), and is given by S Grandhi,J. Zander and R. Yates, “Constrained Power Control,” Wireless PersonalCommunications, Vol. 1, pp. 257-270, 1995 as: $\begin{matrix}{{p_{i}\left( {k + 1} \right)} = {\min\left\{ {{\frac{\gamma_{i}^{tar}}{\gamma_{i}(k)}{p_{i}(k)}},p^{\max}} \right\}}} & (1)\end{matrix}$

U.S. Pat. No. 6,070,086 describes a method for closed loop powercontrol. A closed loop power control unit is coupled to respectivecell-site transmitter/receiver comprising: means for measuring Eblo (theratio of signal energy per bit to the interference power spectraldensity), means for generating power adjustment commands correspondingto deviation in corresponding cell-site Eblo measurement a predeterminedEblo level; wherein the coherent detection schemes are used for reverselink (mobile to cell) in the cellular mobile telephone system andwherein it is assumed that the mobile station is capable of receivingthe power adjustment commands and adjusts the transmission signal powerin correspondence to the power adjustment commands.

The control of power levels of signals transmitted from devices to basestations may be either centralized or distributed. In centralized powercontrol techniques, a single controller determines the power level foreach device in the cell, and communicates that level to each device.Centralized control is advantageous in that a desired CIR level can beachieved immediately since the centralized controller has informationabout devices in contact with the base station (e.g. about which deviceswill terminate or initiate communications in a time interval).Centralized control, however, needs all the information of all thedevices (including all the link gain) and involves the addedinfrastructure of a central control mechanism thereby resulting in addednetwork vulnerability due to the single point of control. Centralizedcontrol schemes have been described in J. Zander, “Performance ofoptimum transmitter power control in cellular radio systems,” IEEETrans. Veh. Technol., Vol. 41, pp. 57-62, February 1992; and, Grandhi etal., “Centralized power control in cellular radio systems,” IEEE Trans.Veh. Technol., Vol. 42, No. 4, pp. 466-468, November 1993. Distributedcontrol, in contrast, only local information is needed for each deviceand can be an iterative approach in which power levels are adjustedbased on feedback from the devices.

Recent work has therefore emphasized distributed, or local, control. Ina distributed power control network, the power level of each device isguided using only local measurements, so that eventually all basestations meet any specified CIR requirements. Such power control methodstypically adjust the power levels in communications devices based on adetermination of a mean (which is a first order statistic) of theinterference level at a base station. See, e.g., J. Zander, “DistributedCochannel Interference Control in Cellular Radio Systems,” IEEE Trans.Vehic. Tech. Bol. 41(3), pp. 305-311, August 1992; G. J. Foschini and Z.Mijanic, “A Simple Distributed Autonomous Power Control Algorithm andits Convergence,” IEEE Trans. Vehic. Tech, Vol. 42 No. 4), pp. 641-646,November 1993; and S. V. Henly, “Capacity In A Two Cell Special SpectrumNetwork,” 30^(th). Annual Conference on Communication, Control andComputing, Allerton House, Monticello, Ill., pp. 426-435, (1992).Distributed control schemes can use only a user's own link gain orCIR/SIR to determine power control, as Gandhi, et al., “DistributedPower Control In Cellular Radio Systems,” IEEE Trans. Comm., Vol. 42,No. 2/3/4, pp. 226-228, February/March/April 1994.

U.S. Pat. No. 5,956,649 describes a method and apparatus which uses aset of parameters characterizing an interference signal at a base unitfor determining power levels for signals transmitted from acommunication device to the base unit. The set of parameters comprisessecond or higher order statistics characterizing the interferencesignal, and the parameters are used to determine a desired power levelfor signals received at the base unit. The desired power level iscommunicated to a communications device via a pilot signal transmittedby the base unit at a predetermined level. The predetermined level andthe power of the received pilot signal are used to compute a path gainbetween the base unit and communications device. The path gain anddesired power level are then used to determine the power level ofsignals transmitted from the communications device to the base unit.

It is desirable to provide an improved closed-loop SIR based powercontrol scheme.

SUMMARY OF THE INVENTION

The present invention relates to a method and system for closed looppower control in a mobile communication system. Error is determinedbetween a target signal-to-interference (SIR) and a received SIR. Apower control scheme at a power controller of a base station determinesa power change that is proportional to the error. Gain of the powercontroller is selected to achieve minimum error at a subsequent timeinstant.

The power controller is an estimation based controller and determines anestimate of channel variations. A discrete time H∞ filter can be used asthe estimator. Optimally the estimate and prediction of error between atarget SIR value and the actual SIR goes to zero in one step. Multiplesteps may be needed for the estimator to provide accurateestimations/predictions, for example the estimator may need three tofour steps to converge and the error can go to zero in four to fivesteps.

In all wireless systems, it is desirable that all mobiles transmit withsmall power as long as they maintain required service quality measuredby the SIR. When the channel quality of a mobile user, denoted user i,becomes poor, mobile user i will raise its transmission power to reachthe desired SIR. This introduces more interference to the system, otherusers that will raise their transmission power to maintain their targetSIR. In a heavily loaded wireless system, all mobiles will soon end upat their maximum transmission power. This phenomenon is described as the“party effect” and should be prevented. The present invention provides amethod for providing joint minimization of SIR error and mobile'stransmission power. This joint minimization of the squared error betweenthe SIR target and the actual SIR value plus the square of thetransmission power to achieve improved performance.

The invention will be more fully described by reference to the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a method for closed loop power control in awireless system.

FIG. 2 is a schematic diagram of a wireless system.

FIG. 3 is a schematic diagram of base station behavior in the closedloop power control method.

FIG. 4 is a schematic diagram of closed loop power control architecturein a 3-G system.

FIG. 5A is a schematic diagram of a closed loop power control system ofthe present invention.

FIG. 5B is a schematic diagram of the closed loop power control systemincluding an estimator.

FIG. 5C is a schematic diagram of the closed loop power control systemfor N links.

FIG. 6 is a schematic diagram of a H∞ filter.

FIG. 7 is a flow diagram of estimator using the discrete time H∞ filter.

FIG. 8A is a schematic diagram of a frame format for a prior art CDMApower control scheme.

FIG. 8B is a schematic diagram of a frame format for the power controlscheme of the present invention.

FIG. 9 is a schematic diagram of a SIMULINK model of the closed looppower control system of the present invention for a two user system.

FIG. 10 is a signal comparison of power evolution for one user for thepower scheme of the present invention, DCPC and IS-95.

FIG. 11 is a signal comparison of error performance for one user for thepower scheme of the present invention DCPC and IS-95.

FIG. 12 is a signal comparison of performance of joint minimization ofSIR error and transmission power for varying weighting parameters andIS-95 with 0.5 dB.

FIG. 13 is a signal comparison of performance of joint minimization ofSIR error and transmission power for varying weighting parameters andIS-95 with 0.5 dB.

DETAILED DESCRIPTION

Reference will now be made in greater detail to a preferred embodimentof the invention, an example of which is illustrated in the accompanyingdrawings. Wherever possible, the same reference numerals will be usedthroughout the drawings and the description to refer to the same or likeparts.

FIG. 1 is a flow diagram of a method for closed loop power control in awireless system 10 in accordance with the teachings of the presentinvention. In block 12, a target signal to interference ratio (SIR) isdetermined. For example, the target SIR can be set by mapping thedesired Frame Error Rate (FER) to the target SIR or with a predeterminedvalue.

In block 14, an estimate of a received SIR is determined at the basestation. Error between the estimated received SIR and the target SIR isdetermined in block 16. In block 18, the power change is determined tobe proportional to the error. The optimal gain of the controller forimplementing the power change at the base station is selected to achieveminimum error at the next time instant, in block 19.

FIGS. 2-5 illustrate system for implementing closed loop power controlin a wireless system 20 in accordance with the present invention. FIG. 2is a schematic diagram of a wireless system 20. Mobile units 21 areconnected by respective channels 22 to base station 23.

FIG. 3 is a schematic diagram of behavior of base station 23. Signal 24is received at matched filter 25 and is fed to rake combiner 26. Outerloop 27 comprises Viterbi decoder 28, frame error detector 29, and outerloop control 30. The frame error rate (FER) is estimated by frame errordetector 29. Outer loop control 30 maps the estimated FER to a targetSIR value. Inner loop 31 comprises SIR measurement estimator 32 andinner loop control 33. SIR measurement estimator 32 determines estimatedSIR. Inner loop control 33 compares the estimated SIR to the target SIRand determines power control scheme 34. Preferably, outer loop 27 runsat a longer time scale than inner loop 31. For example, outer loop 27can run at about a 10 ms time scale and inner loop 31 can run at about a0.625 ms time scale.

FIG. 4 illustrates a schematic diagram of closed loop power controlarchitecture in a 3-G system. Power control scheme 34 uses input fromSIR measurement estimator 32 and the target SIR value to determine powercontrol command 35. Power control scheme 34 of base station 23 forwardspower control command 35 over channel 22 to mobile unit 21.

FIG. 5A is a block diagram of a closed loop power control system 40 inaccordance with the teachings of the present invention. The transmissionpower is determined to be proportional to the error between the actualSIR, δ, and the target SIR, δ_(i) ^(tar). The transmission power changefrom time step k to k+1 can be defined asΔp _(i)(k+1)=p _(i)(k+1)−p _(i)(k)   (1)where p_(i)(k) and p_(i)(k+1) are the transmission powers from mobile ito the base station at time instants k and k+1, respectively. The errorbetween the actual SIR, γ_(i)(k), and the target SIR, γ_(i) ^(tar), isdenoted by e_(i)(k) for mobile i at time instant k and is represented bye _(i)(k)=γ_(i) ^(tar)−γ_(i)(k)   (2)

The power control algorithm of the present invention can be representedbyΔp _(i)(k+1)=α_(i)(k)e _(i)(k)   (3)where α_(i)(k) of block 42 is the gain to be determined through anoptimization procedure. For example, the optimization procedure candetermine the minimization of the square of the error to theoreticallyreduce the error to zero in each discrete-time instant k. The SIR γ_(i)^((k)) of an active link is subtracted from target SIR γ_(i) ^(tar) inadder 41 to generate error e_(i)(k). The gain α_(i)(k) is determined inblock 42 from error e_(i)(k). Gain α_(i)(k) is applied in adder 43 withtransmission power p_(i)(k) to select the gain to achieve the minimumerror of transmission power p_(i)(k+1). Block 44 determines channelvariation δ_(i)(k) which can be used in determining gain α_(i)(k).

A block diagram of the power control scheme of the present inventionwith an estimator is shown in FIG. 5B. The SIR of an active link frommobile station i to base station n in a wireless system is defined as$\begin{matrix}{\gamma_{ni} = \frac{L\quad h_{ni}p_{i}}{{\sum\limits_{j \neq i}{h_{nj}p_{j}}} + \sigma^{2}}} & (4)\end{matrix}$where h_(ni) is the link gain from mobile station L to base station n asshown in block 46. L is the processing gain as shown in block 47. Forexample L can be the processing gain in a spread spectrum wirelesssystem. For example, in CDMA 2000, L is 64 or 128 or 256. σ² is thebackground noise at the base station. The denominator can be denoted byI_(i), which is the received interference, and SIR can be rewritten as$\begin{matrix}{\gamma_{ni} = \frac{L\quad h_{ni}p_{i}}{I_{i}}} & (5)\end{matrix}$

Assuming that a mobile only transmits to one base station during thetime of power control, then γ_(ni) can be simplified as γ_(i). Thechannel variation is represented by${\delta_{i} = \frac{L\quad h_{ni}}{I_{i}}},$which is estimated and predicted in the power control scheme usingestimator 50, function ƒ₁( ) of block 51, function ƒ₂( ) of block 52 andfunction ƒ( ) of block 53. {circumflex over (δ)}_(i)(k) and {circumflexover (δ)}_(i)(k+1) denote the estimated/predicted value respectively of{circumflex over (δ)}_(i)(k) and {circumflex over (δ)}_(i)(k+1). Thedistributed power control scheme can be determined by minimizing thesquare of error between the target SIR and the actual value.

ƒ( ) of block 53 is defined as $\begin{matrix}{{f\left( {x_{1},x_{2}} \right)} - {\left\{ {{\begin{matrix}\frac{1}{x_{1}} \\0\end{matrix}\left( {1 - \frac{\gamma_{i}^{tar}}{e_{i}(k)}} \right)} + {\frac{1}{x_{2}}\frac{\gamma_{i}^{tar}}{e_{i}(k)}}} \right\}\quad\begin{matrix}{{{if}\quad{e_{i}(k)}} \neq 0} \\{{{if}\quad{e_{i}(k)}} = 0}\end{matrix}}} & (7)\end{matrix}$ƒ₁(x) of block 51 is 10log₁₀(x) and ƒ₂(x) of block 52 is 10^((x)/10)which are converting functions between W and dBW. W is the unit ofpower, Watt. Accordingly, feedback loop 55 computes the optimal gain ofthe controller based on the estimated/predicted channel variations.

The performance criterion at time instant k can be defined as$\begin{matrix}\begin{matrix}{{{J_{i}(k)} = {\begin{matrix}\min \\{\alpha_{i}(k)}\end{matrix}\left( {e_{i}\left( {k + 1} \right)} \right)^{2}}},} & {{i = 1},2,\ldots\quad,{N.}}\end{matrix} & (8)\end{matrix}$

The optimal gains of the power control algorithm to minimize J_(i)(k) atevery time instant k are $\begin{matrix}{{\alpha_{i}^{opt}(k)} = \left\{ {\begin{matrix}\frac{i}{\alpha_{i}(k)} \\0\end{matrix}\left( {1 - \frac{\gamma_{i}^{tar}}{e_{e}(k)} + {\frac{1}{\delta_{i}\left( {k + 1} \right)}\quad\frac{\gamma_{i}^{tar}}{e_{i}(k)}\quad\begin{matrix}{{{if}\quad{e_{i}(k)}} \neq 0} \\{{{if}\quad{e_{1}(k)}} = 0}\end{matrix}}} \right.} \right.} & (8.1)\end{matrix}$where $\begin{matrix}{{{\delta_{i}(k)} = {\frac{L\quad{h_{ni}(k)}}{I_{i}(k)} > 0}},} & {{\delta_{i}\left( {k + 1} \right)} = {\frac{L\quad{h_{ni}\left( {k + 1} \right)}}{I_{i}\left( {k + 1} \right)} < 0}}\end{matrix}$and h_(ni)(k),h_(ni)(k+1,I_(i)(k+1)>0∀k.The minimal value of the performance criterion is zero when the optimalgain α_(i) ^(opt)(k) is applied.The optimal distributed power control scheme is given by $\begin{matrix}{{p_{i}\left( {k + 1} \right)} = \left\{ \begin{matrix}p^{\min} & {{{if}\quad{{\hat{\delta}}_{i}\left( {k + 1} \right)}} > \frac{\gamma_{i}^{tar}}{p^{\min}}} \\p^{\max} & {{{if}\quad{{\hat{\delta}}_{i}\left( {k + 1} \right)}} < \frac{\gamma_{i}^{tar}}{p^{\max}}} \\{{{p_{i}(k)} + {{{\hat{\alpha}}_{i}(k)}\quad{e_{i}(k)}}} = \frac{\gamma_{i}^{tar}}{{\hat{\delta}}_{i}\left( {k + 1} \right)}} & {o.w.}\end{matrix} \right.} & (9)\end{matrix}$where the gain applied to the power control scheme {circumflex over(α)}_(i)(k) is defined by $\begin{matrix}{{{\hat{\alpha}}_{i}(k)} = \left\{ {{\begin{matrix}\frac{1}{{\hat{\delta}}_{i}(k)} \\0\end{matrix}\left( {1 - \frac{\gamma_{i}^{tar}}{e_{i}(k)}} \right)} + {\frac{1}{{\hat{\delta}}_{i}\left( {k + 1} \right)}\quad\frac{\gamma_{i}^{tar}}{e_{i}(k)}\quad\begin{matrix}{{{if}\quad{e_{i}(k)}} \neq 0} \\{{{if}\quad{e_{i}(k)}} = 0}\end{matrix}}} \right.} & (10)\end{matrix}$

Equation (9) is consistent with Pontriagin's minimum principle such thatin constrained optimization, optimal solutions lie on the boundary.Closed loop power control system 40 can be implemented at base station23, shown in FIG. 2.

FIG. 5C is a schematic diagram of a distributed power control systemwith estimations for N links. The present invention provides a methodfor providing joint minimization of SIR error and transmission error SIRjoint minimization of squared error between the SIR target and theactual SIR value plus the square of the transmission power to achieveimproved performance.The performance criterion at time instant k is defined as$\begin{matrix}{{{J_{i}(\quad k)} = \quad{\begin{matrix}\min \\{\alpha_{i}(k)}\end{matrix}\left\lbrack \quad{{\rho_{1}\left( {e_{i}\left( {k + 1} \right)} \right)}^{2} + \quad{\rho_{2}\left( {p_{i}\left( {k + 1} \right)} \right)}^{2}} \right\rbrack}},\quad{i = \quad 1},\quad 2,\quad\ldots\quad,\quad{N\quad.}} & (11)\end{matrix}$where ρ₁>0 and ρ₂>0 are weighting parameters.In this embodiment ƒ( ) is defined as $\begin{matrix}{{f\left( {x_{1},x_{2}} \right)} = \left\{ {\begin{matrix}\frac{1}{x_{1}} \\0\end{matrix}\left( {1 - \frac{\gamma_{i}^{tar}}{e_{i}(k)} + {\frac{\rho_{1}x_{2}}{{\rho_{1}x_{2}^{2}} + \rho_{2}}\frac{\gamma_{i}^{tar}}{e_{i}(k)}\quad\begin{matrix}{{{if}\quad{e_{i}(k)}} \neq 0} \\{{{if}\quad{e_{i}(k)}} = 0}\end{matrix}}} \right.} \right.} & (12)\end{matrix}$ƒ_(i)(x)=10log₁₀(x) and ƒ₂(x)=10^((x)/10) are converting functionsbetween W and dBW.The optimal distributed power control scheme by joint optimization isgiven by $\begin{matrix}{{p_{i}\left( \quad{k + \quad 1} \right)} = \quad\left\{ \quad\begin{matrix}p^{\min} & {{{if}\quad{{\hat{\delta}}_{i}\left( {k + 1} \right)}} > \frac{\gamma_{i}^{tar}}{p^{\min}}} \\p^{\max} & {{{if}\quad\frac{{\rho_{1}\left( {{\hat{\delta}}_{i}\left( {k + 1} \right)} \right)}^{2} + \rho_{2}}{\rho_{1}\left( {{\hat{\delta}}_{i}\left( {k + 1} \right)} \right)}} < \frac{\gamma_{i}^{tar}}{p^{\max}}} \\{{p_{i}(k)} + {{{\hat{\alpha}}_{i}(k)}\quad{e_{i}(k)}}} & {o.w.}\end{matrix} \right.} & (13)\end{matrix}$where {circumflex over (α)}_(i)(k) is defined in the following equation:$\begin{matrix}{{{\hat{\alpha}}_{i}(k)} = \left\{ {{\begin{matrix}\frac{1}{{\hat{\delta}}_{i}(k)} \\0\end{matrix}\left( {1 - \frac{\gamma_{i}^{tar}}{e_{i}(k)}} \right)} + {\frac{\rho_{1}{{\hat{\delta}}_{i}\left( {k + 1} \right)}}{{\rho_{1}\left( {{\hat{\delta}}_{i}\left( {k + 1} \right)} \right)}^{2} + \rho_{2}}\frac{\gamma_{i}^{tar}}{e_{i}(k)}\quad\begin{matrix}{{{if}\quad{e_{i}(k)}} \neq 0} \\{{{if}\quad{e_{i}(k)}} + 0}\end{matrix}}} \right.} & (14)\end{matrix}$The power control y_(i)(k) of each link is added in adder 56.

Estimator 50 can be a conventional filter. In general, fluctuation ofδ(k) and δ(k+1) do not have a Gaussian distribution. Accordingly,filters which do not use knowledge of the statistics of the processand/or noise are preferable. The H∞ filter which does not require anyknowledge of the statistics of system and measurement disturbances ispreferable for implementation of the present invention. FIG. 6illustrates of schematic diagram of a discrete H∞ filter 70 which can beused with closed loop power control system 40.

Consider the following discrete-time systemx(k+1)=A(k)x(k)+B(k)w(k), x(0)=x ₀   (15)with the measurementsz(k)=C(k)x(k)+v(k)   (16)where x(k) is the state vector, w(k) is the process noise vector, z(k)is the measurement vector and v(k) is the measurement noise vector.A(k), B(k) and C(k) are matrices of appropriate dimensions. Themeasurement history is defined as z(k), k=0, 1, . . . , N−1. Theestimate of {circumflex over (x)}(k) can be computed based on themeasurement history up to k−1. The H∞ filter provides an uniformly smallestimation error x(k)−{circumflex over (x)}(k) for any w(k), v(k)εl₂ andx₀εR^(n). The performance criterion is given by $\begin{matrix}{J = \frac{\sum\limits_{k = 0}^{N - 1}{{{x(k)} - {\overset{\Cap}{z}(k)}}}_{Q{(k)}}^{2}}{{{{x(0)} - {\overset{\Cap}{x}(0)}}}_{P - {1{(0)}}}^{2} + {\sum\limits_{k = 0}^{N - 1}{{w(k)}}_{W^{- 1}{(k)}}^{2}} + {{v(k)}}_{V^{- 1}{(k)}}^{2}}} & (17)\end{matrix}$where Q(k), W(k) and V(k) are weighting matrices. Accordingly, theoptimization problem is $\begin{matrix}{{\begin{matrix}\sup \\{{Q(k)},{W(k)},{V(k)}}\end{matrix}J} < \frac{1}{\lambda}} & (18)\end{matrix}$where λ is a prescribed level of noise attenuation. The resulteddiscrete-time H∞ filter is given by{circumflex over (x)}(k+1)=A(k){circumflex over(x)}(k)+K(k)(z(k)−C(k){circumflex over (x)}(k))   (19)where K(k) is the optimal gain of the H∞ filter and is computed by thefollowing equationK(k)=A(k)P(k)(I−λQ(k)P(k)+C ^(T)(k)V ⁻¹(k)C(k)P(k))⁻¹ C ^(T)(k)V ⁻¹(k)  (20)where P(k) is the unique positive definite solution of the followingdiscrete-time Riccati equationP(k+1)=A(k)P(k)(I−λQ(k)P(k)+C ^(T)(k)V ⁻¹(k)C(k)P(k))⁻¹ A^(T)(k)+B(k)W(k)B ^(T)(k)   (21)with P(0)=P₀.

FIG. 7 is a flow diagram of estimation using the discrete time H∞filter. In block 60, all the parameters in the optimization criterion(Q_(i),V_(i),W_(i),λ),δ_(i)(0)=δ_(i0) are initialized and matrixP_(i)(0)=P_(i0) for k=0.In block 61, the solution of the following scalar discrete-time Riccatiequation is computed from $\begin{matrix}{{K_{i}(k)} = \frac{P_{i}(k)}{\left( {1 - {\lambda\quad Q_{i}{P_{i}(k)}} + \frac{P_{i}(k)}{V_{i}}} \right)V_{i}}} & (23)\end{matrix}$In block 62 the optimal gain K_(i)(k) of the H∞ filter is computed from$\begin{matrix}{{{\hat{\delta}}_{i}\left( {k + 1} \right)} = {{{\hat{\delta}}_{i}(k)} + {{K_{i}(k)}\left( {{y_{i}(k)} - {{\hat{\delta}}_{i}(k)}} \right)}}} & (24)\end{matrix}$In block 63 estimated and predicted channel variation ({circumflex over(δ)}_(i)(k),{circumflex over (δ)}_(i)(k+1)) is computed through thediscrete-time H∞ filter, which is given by{circumflex over (δ)}_(i)(k+1)={circumflex over (δ)}_(i)(k)+K _(i)(k)(y_(i)(k)−{circumflex over (δ)}_(i)(k))   (24)In block 64, k is incremented as. k=k+1, and blocks 61-63 are repeated.

FIG. 8A represents implementation in a frame format of a prior art CDMApower control scheme. Frame 70 includes pilot symbol 72 and powercontrol TPC (+1, −1) 73 and payload 74. The overhead is 16 or 32bits/frame. FIG. 8B represents implementation of a frame format for thepower control scheme of the present invention. Power control TPL is 3 to5 bits. Accordingly, the overhead is 3 to 5 bits/frame.

A simulation of a CDMA system was performed to demonstrate theeffectiveness of optimal power control method 10 as compared toconventional existing power control schemes. Four users in a singlehexagonal cell were used in the simulation. The operating frequency wasabout 1.9 GHz and the bandwidth of each channel is assumed to be about1.23 HMz, in accordance to the CDMA 2000 standard as described in ITU,RTT 45.5, CDMA 2000 standard. The data rate was set at 9600 bps and theprocessing gain was set to 32. The target SIR is 10 dB, whichcorresponds to the bit error rate (BER) being less than 10⁻⁴. The SIRincludes the processing gain, which is denoted by $\frac{E_{b}}{N_{0}}$in the standard IS-95B described in TIA/EIA Interim Standard-95B: Mobilestation-base station compatibility standard for dual-mode widebandspread spectrum cellular system, TIA, 1998 and CDMA 2000 ITU, RTT 45.5,CDMA 2000 standard, hereby incorporated by reference into thisapplication, where E_(b) is the energy per information bit and N₀ is theinterference power spectral density.

The following assumptions were used in the simulation:

1. The effects of antenna directivity, voice activity factor, and softhandoff were ignored.

2. The minimum and maximum power that can be transmitted by a mobile asdescribed in IS-95B is p_(min)=8 dBm (6.3 mW) and p_(max)=33 dBm (2 W),respectively.

3. The background noise power is 0.5 mW.

4. The transmitted power is updated periodically, every 0.625 msec,which corresponds to 1,600 Hz fast closed-loop power control frequencyas proposed in IMT-2000.

5. The antenna of the base station and all mobiles are omnidirectional.

6. The location of the mobiles are assumed to be uniformly distributedin a cell.

7. It is assumed that the link gains have the following formh _(ni)(k)=d _(ni) ⁻⁴(k)A _(ni)(k)B _(ni)(k)   (15)where d_(ni)(k) is the distance from the ith mobile to the nth basestation at time instant k_(i), A_(ni) is a log-normal distributedstochastic process, and B_(ni) is contributed by Rayleigh fading.d_(ni)(k), A_(ni), and B_(ni)(k) change in time since they are functionsof mobile velocity and frequency. Using method 10, the desired SIRachieved in less than 10 steps (6.25 msec), such that d_(ni)(k) andA_(ni) barely change during this short time period.

8. It is assumed that the cell diameter is 2 km and D_(ni)(k) is a 2-Duniformly distributed random variable, as described in G. Stuber,Principles of Mobile Communication, Kluwer Academic Publishers, 1996,hereby incorporated by reference.

9. It is assumed that the standard deviation of A_(ni) is 8.

10. The speed of the mobiles range from about 0.5 mph to about 75 mph.The maximum Doppler frequency is 80 Hz, which correspond to 60 mphmobile speed. B_(ni)(k) is simulated for the 4-user case by using afiltered Gaussian noise model.

11. The mean value of the link gains are used in the simulation.

FIG. 9 illustrates a SIMULINK model of closed loop power control system40. FIGS. 10-13 show results of the simulation. The power evolution ofmethod 10 is compared with DCPC and IS-95 in FIG. 10. The errorperformance of method 10 is compared with DCPC and IS-95 in FIG. 11. Itis shown that when the channel is stationary (the mean values of thechannels are constants in the simulation), method 10 saves at least 2frames, which frames are lost when applying the other two conventionalpower control schemes. When channel changes frequently, the estimator ofmethod 10 provides good tracking of the channel quality and the proposedoptimal power control scheme and provides accurate value of poweradjustment immediately. The DCPC scheme and the IS-95 scheme do notconverge fast enough to give appropriate power change.

A comparison of the power evolution and SIR error performance using apower control scheme of method 10 by applying joint minimization of SIRerror and transmission power, using the power control scheme of method10 minimizing SIR error only, and IS-95, are shown in FIG. 12 and FIG.13. It is shown that with the panelty on mobile's transmission power,the method of the joint minimization scheme of the present inventionuses less mobile's transmission power, while keeping small SIR error.The SIR error performance is the same as using the optimal power controlscheme which minimizes SIR error only in the practical CDMA systemssince there is quantization effect at the mobile when receiving powercontrol command. It is shown that method 10 which minimizes SIR erroronly can not achieve zero SIR error due to the quantization effect.Method 10 including joint minimization in the power control scheme hasthe advantage of saving the mobile's transmission power, whilemaintaining similar level of SIR error. With less mobile's transmissionpower, the method 10 of the present invention including the jointminimization scheme has much better SIR error performance than the IS-95scheme. When there is no panelty on mobile's transmission power bysetting p₂=0, the joint minimization scheme, of the present invention,reduces to the optimal power control scheme which minimizes SIR erroronly.

It is to be understood that the above-described embodiments areillustrative of only a few of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can be readily devised inaccordance with these principles by those skilled in the art withoutdeparting from the spirit and scope of the invention.

1. A transmission power control method for a mobile communication systemcomprising the steps of: determining a target Signal-to-InterferenceRatio (SIR); estimating a received SIR of a base station; determiningerror between the received SIR and the target SIR; and determining apower control scheme at a power controller of the base station todetermine a power change for the transmission power of a mobile to thebase station in proportion to the error; and selecting a gain of thepower controller to achieve minimum error at a subsequent time instant.2. The method of claim 1 wherein said gain is determined as aminimization of a square of the error.
 3. The method of claim 1 furthercomprising the step of: estimating channel variation and using saidestimated channel variation in said step of selecting said gain. 4.(canceled)
 5. (canceled)
 6. (canceled)
 7. (canceled)
 8. (canceled) 9.The method of claim 1 wherein said power change is transmitted from saidbase station to said mobile in a frame format on a downlink from saidbase station to said mobile.
 10. The method of claim 9 wherein overheadof said power change is in the range of 3 to 5 bits per frame.
 11. Atransmission power control system for a mobile communication systemcomprising: means for determining a target Signal-to-Interference Ratio(SIR); means for estimating a received SIR of a base station; means fordetermining error between the received SIR and the target SIR; and meansfor determining a power control scheme at a power controller of the basestation to determine a power change for the transmission power of amobile to the base station in proportion to the error; and means forselecting a gain of the power controller to achieve minimum error at asubsequent time instant.
 12. The system of claim 11 wherein said gain isdetermined as a minimization of a square of the error.
 13. The system ofclaim 11 further comprising: means for estimating channel variation andusing said estimated channel variation in said step of selecting saidgain.
 14. (canceled)
 15. (canceled)
 16. (canceled)
 17. (canceled) 18.(canceled)
 19. The system of claim 11 wherein said power change istransmitted from said base station to said mobile in a frame format on adownlink from said base station to said mobile.
 20. The system of claim19 wherein overhead of said power change is in the range of 3 to 5 bitsper frame.